Petrogenetic realtionships between relatively potassic and relatively sodic alkaline rocks from the Spanish Peaks area near Walsenburg, Colorado

Abstract

Alkaline lamprophyre rocks crop out as sills and sub parallel dikes near the Spanish Peaks, CO. Lamprophyre dikes and sills north, south, and west of Spanish Peaks are moderately mafic (~7 wt.% MgO), basic (~49 wt.% SiO2 ), and range from relatively sodic (Na2O>K2O) to relatively potassic (K2O>Na2O). Previous geochronologic and isotopic research on rocks investigated in this study, indicate that sodic and potassic magmatism originated at approximately the same time and from isotopically similar mantle sources. Rock types include minettes and sannaites, which have phlogopite or amphibole ± clinopyroxene ± olivine phenocrysts and groundmass potassium feldspars. Rare diopside grains in potassic rocks have greenish, Fe- and Al-rich cores found worldwide in other potassium-rich, mafic igneous rocks. Major and trace element abundance data from this study indicate that the two magma series formed independently from distinct sources.